1. Field of Endeavor
The invention relates to the field of material technology, and more particularly to a method for surface treatment of ferritic/martensitic 9-12% Cr steels which are used predominantly for the production of components employed in steam power stations. These steels are exposed to high temperatures (typically 600 to 650° C.) and therefore have to be protected against damage, that is to say loss of quality, as a result of oxidation and subsequent flaking.
2. Brief Description of the Related Art
It is known that austenitic steels which are highly alloyed, inter alia, with chromium, are employed for superheater and intermediate superheater tubes in power stations. It is known of austenitic steels that an improved oxidation behavior of the material can be achieved by means of a cold forming of the surface, for example by bombarding the surface of the steel with small particles of a carbon steel at high velocity (=shot peening). The reason for this is a martensitic transformation of the surface thus treated, in which a large number of grain boundaries arise, which, in turn, enable the chromium present in the steel to migrate onto the surface and there form chromium oxides which then protect the material against further oxidation (see D. Caplan, Corr. Science 6 (1966), 509 and Y. Minami, NKK Tech. Rev. 75 (1996),1).
Furthermore, ferritic/martensitic steels with approximately 9-12% Cr are known which are used predominantly for tubes, valves and housings. Mention may be made as examples of these of the steels P92 (chemical composition in % by weight: 0.12 C, 0.5 Mn, 8.9 Cr, 0.4 Mo, 1.85 W, 0.2 V and the rest iron and unavoidable impurities) and also E911 (chemical composition in % by weight: 0.11 C, 0.35 Mn, 0.2 Si, 9.1 Cr, 1.01 Mo, 1.00 W, 0.23 V, and the rest iron and unavoidable impurities). These ferritic/martensitic steels, because of their chemical composition, are generally less oxidation-resistant than austenitic steels, but they usually likewise have to withstand high temperatures of up to 620° C. in modern power stations. To protect steels of this type against harmful oxidation, therefore, special coatings were developed (A. Agüero, R. Muelas, Mat. Sci. Forum, Vol. 461 (1994), 957). These coatings have the disadvantage, on the one hand, of being costly and, on the other hand, of not always being reliable. If coatings are applied, there is always the need for heat treatment or even several heat treatments which, in turn, are costly and time-consuming, particularly because very large components have to be heat-treated in power station construction. Alternatives, above all simpler possibilities for oxidation protection for ferritic/martensitic steels of this type have therefore already been desired for a long time.
In contrast to austenitic steels, however, the known shot peening, in the case of ferritic/martensitic steels, does not have the positive effect described above because of the different structure.
Nevertheless, H. Haruyama, H. Kutsumi, S. Kuroda and F Abe, Proc. of EPRI Conf., (2004), 659-667, reported a slight increase in the oxidation resistance of steels of this type in steam when these have been shot-peened with pure chromium particles before temperature and steam loading and have subsequently been subjected to heat treatment at 700° C. The latter, however, has the disadvantage of being highly cost-intensive and is undesirable in terms of the required structure in power station construction.